Body organ threshold analyzer

ABSTRACT

A current control device having particular application as a body organ threshold analyzer is described comprising pulse current selection means by which a desired output pulse current from a pulse generating means, such as a cardiac pacer, can be selected, pulse current control means by which the pulse current can be controlled, and a pulse current sensing and correcting means by which the pulse current is sensed and when deviations from the selected current occur, sends corrective signals to the current control means.

United States Patent [1 1 Herrmann Nov. 6, 1973 BODY ORGAN THRESHOLDANALYZER [75] Inventor: Cal C. Herrmann, New Shewsbury,

[73} Assignee: ESB Incorporated, Philadelphia, Pa.

[22] Filed: May 5, 1971 [21] Appl. No.: 140,361

[52] US. Cl. 128/419 P, 128/421, 307/264,

323/4, 323/22, 328/172 [51] Int. Cl A6ln 1/36 [58] Field of Search128/419 C, 419 E,

[56] References Cited UNITED STATES PATENTS 2,978,630 4/1961 La Tour323/9 3,255,402 6/1966 Vollnhals.... 323/9 3,513,378 5/1970 Kemper 323/93,548,294 12/1970 Houghton 323/9 3,523,539 8/1970 Lavezzo et a1. 128/419P F l PULSE :o L GENERATOR 3,648,708 3/1972 Haevi 128/422 3,625,20112/1971 Murphy, Jr 128/419 P 3,554,198 l/197l Tatolan et a]. 128/419 POTHER PUBLICATIONS Chardack et al., Surgery Vol. 48, No. 4, Oct. 1960pp. 643-654.

Primary Examiner-William E. Kamm AttorneyRobert H. Robinson, Raymond L.Balfour, Anthony J. Rossi and Thomas A. Lennox [57] ABSTRACT A currentcontrol device having particular application as a body organ thresholdanalyzer is described comprising pulse current selection means by whicha desired output pulse current from a pulse generating means, such as acardiac pacer, can be selected, pulse -current control means by whichthe pulse current can be controlled, and a pulse current sensing andcorrecting means by which the pulse current is sensed and whendeviations from the selected current occur, sends corrective signals tothe current control means.

1 Claim, 2 Drawing Figures PAIENIEBImv ems 3.769.986

I PULSE L- lo I GENERATOR J L .4 I+I HI:

I |V I4 I I CURRENT I SELECTION q i I CURRENT I I SENSING CURRENT ICONTROL RETURN- I ELECTRODE l' I ISTIMULATING 20 {ELECTRODE I \J v F 7g.I

'r'" 1 PULSE IO GENERATOR INVENTOR.

CAL C. HERRMANN 1 BODY ORGAN THRESHOLD ANALYZER BACKGROUND OFTI-IEINVENTION l. Field of the Invention v This invention relates to acurrent control device. In particular, it relates to a currentcontroldevice or threshold analyzer for controlling the output of a pulsegenerating means such as a body organ stimulating device andfordetermining the threshold requirements for organ stimulation asmeasured on the patient. The invention will be described for the mostpart as applied to'cardiac pacers and cardiac thresholds analysis as itis in this area where most of the organ stimulation work has beendeveloped. However, the invention may be readily adapted for use withstimulators for many other body organs, muscular tissue, etc.

2. Description of the Prior Art S The extension of human life by the useof implanted heart stimulating devices has been carried out with greateffectiveness for approximately a decade. In order to simplify theimplanting operation, in order to be sure'that the heart stimulatingelectrodes are properlylocated, and particularly to insure that theimplantable stimulator is in proper operating condition, it has beenfound desirable to measure the minimum heart stimulating impulserequired by the patient and compare it to the outputqof the stimulatingdevice with which he will be supplied. Several devices for making thesemeasurements have been heretofore described in the prior art. Thegeneric name of threshold analyzers hasbeen used for such devices andwill be used in the following discussionaln general, these thresholdanalyzers have measured the minimum stimulating requirement in terms offractions of the energy output of the stimulator to-beused with anindividual patient. Thus, with the threshold analyzers available, thereis little opportunity for o btaining statistical information on actualenergy requirements; Further, when a particular stimulator-electrodecombination does not appearto have a sufficient factor of safety for areasonably long implant, there is no conclusive way with the prior artdevices'to determine whether" it is'the electrode system or thestimulator device that is at fault.-

trol means. In general, these devices are large or contain numerouscomponents.

SUMMARY OF THE INVENTION The present invention provides a simplifiedcurrent control device having particular application as a body organstimulator threshold analyzer means comprising: a current selectionmeans by which the pulse current of a body organ stimulator can beselected, a current controlling means by which the pulse current can becontrolled, and a current sensing means by which the pulse currentcan besensed and when deviations from the selected current occur sendscorrective signals to the control means. The sensing and correctingmeans include a transistor the forward base emitter junction potentialof whichserves as a reference potential against In my co-pendingapplicationSer. No. 140,360, filed May 5,1911, a form of thresholdanalyzer comprising a completecircuit for determining the peak voltageand current of a stimulating pulse isucombined with a heart stimulatorto enable a surgeon to make, complete and exhaustive tests of thestimulating electrodes when implanted in the patient as well as enablingthe surgeon to fully check out the implantable stimulator to be usedwith a particularpatient. This form of threshold analyzer is ideal foruse in large hospitals where there may be several stimulator implantoperations in a week. However, for smaller hospitals where implantingoperations may not be held oftener than about onceper month or so or forapplications where a low cost or disposable device is preferred, a lesscostly and less complicated device, yet one that will indicate truecurrent values independent of the current or voltage capabilitiesof thegenerator, is needed.

In general, current controlling devices have been used extensively inthe past. One such device is the iron wire ballast tube. This has thefault of being very slow in its action, requiring several minutes toreach equilibrium and hence, is not suited to pulse type operations.

Certain power supply devices also embody current conwhich deviations ofthe pulse current are measured. A thresholdanalyzer device in accordancewith the invention is small in size and low in cost. A first advantageover prior art threshold analyzers is that the threshold current ascontrolled by the threshold analyzer of the invention is indicateddirectlyin terms of true current values, i.e., milliamperes.

Another advantage of the device when used as a threshold analyzer isthat it requires no external power in its operation. In addition, it isof almost universal application and is suitable for use with mostconventional heart pacers presently available. Further, the currentcontrol device of the invention'provides a low impedance reverse pathwhen not conducting current in a. forward direction. This isparticularly advantageous when using the analyzer with a synchronouspacer in which the natural heart rhythm issensed and returned to thepacer. The total voltage drop across the threshold analyzer of theinvention is low-due to the voltage reference selected so thatcontrolled pulses therefrom have sufficient voltage to give properstimulation in spite of limited voltage available from the implantablestimulating pulse generating means.

BRIEF DESCRIPTION OF THE DRAWINGS,

' FIG". 1 illustrates in block form the several functional portions ofthe threshold analyzerof the invention; and FIG. 2 is a completeelectrical circuit of a particular embodiment of the invention."

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates in-block formthe principal functional portions of the threshold analyzer of theinvention. In this diagram, a stimulating pulse generating means, as forexample a'heart pacer, is indicated at 10, and a stimulating electrodealready implanted in the patient is shown at 12. The heart pacer isadapted to generate a series of .heart stimulating pulses at the nor malpulse rateand of a strength sufficient to properly stimulate the heartof the patient. The stimulating electrode is adapted todeliver thestimulating pulses from the pacer to the heart of the patient in such amanner that the patients heart is stimulated tobeat at the rate providedby the pacer. As described above, the stimulator 10 is intended to beimplanted in the patient after its performance with the implantedelectrode 12 has. been thoroughly analyzed by means of the analyzer.

The analyzer comprises three functional blocks, a stimulating pulsecurrent selection means 14, stimulating current controlling means 16 anda current sensing and corrective means 18. As shown, the currentselection means 14 and the current controlling means 16 are connected inseries between the stimulator l and the electrode 12. The currentsensing means 18 is connected between the current selection means 14 andthe current controlling means 16. The current sensing means senses thepulse current and when the pulse current deviates from the selectedcurrent, its sends corrective signals to the current control means.

The operation of the device can be understood by following its use in atypical implantation operation 'as follows: a typical heart blockpatient, after the'imm'ediate crisis is over, is put on a temporaryexternal heart stimulator. As soon as possible, arrangements are madefor the implantation of a permanent stimulator. An implantablestimulating pulse generator is selected and an output or stimulatingelectrode is selected and located in a suitable cardiac location. Thethreshold analyzer with current selector set for maximum current isconnected between the output of the stimulating generator and theelectrode. In certain cases, i.e., when used with stimulators having anunpolar electrode, it is also necessary to provide a temporary returnlead from the patient to the generator. This is shown at 20. At the sametime that the implantable stimulating circuit is completed, it isnecessary to break the circuit of the temporary external stimulator,otherwise, the heart of the patient would receive a double quantity ofpulses which would, of course, be undesirable. A sufficient time is thenallowed to pass for the heart of the patient to get accustomed to thestimulating pulses from the new source. When this is achieved, thesurgeon, by means of the current selection portion of the analyzer,gradually reduces the pulse current. This is continued until the surgeonobserves through an electrocardiograph or other means that thestimulating current is at the thresholdor the minimum required tostimulate the patient.

A particular feature of the threshold analyzer of the invention is-thatthe currents passed by the analyzer are given in terms of milliampers sothat they are reproduceable and suitable for correlation. If thethreshold current, as determined by the threshold analyzer, is undulyhigher than expected by the surgeon, the surgeon is advised that thestimulating electrode is not properly located or that other problemsexist. The threshold current as determined can be entered on thepatients medical history sheet so as to be available for future checks.

In FIG. 2, a detailed circuit diagram of one form of the thresholdanalyzer is shown. In this diagram represents the stimulating generatorand 12 the implanted electrode. An input terminal 56 to which thegenerator lead is externally connected connects internally with thecurrent selection means 14 comprising a variable resistor R6. in FIG. 2,a fixed resistor R1 is shown in series with R6. The purpose 'of R1 is toprovide a safe limit to the current capabilities of the control.Although Rl is shown as being separate from R6, R1 can be combined withR6 as for instance, by providing a stop to limit the excursion of R6. Itcan also be pointed out that R1 is not a necessary part of thecontroldevice and may be omitted without in any way altering its operation as acurrent controlling device. it is desirable that resistor R6 has alogarithmic resistance characteristic so as to cover a broad range ofresistance values with approximately equal accuracy at all dialsettings.

The secondterminal of R6 connects to the collector of a currentcontrolling transistor Q1 and represents the current control means 14.It is desirable that Q1 has a low saturation voltage,'low emitter-basepotentials, and that in general it will operate in a voltage range lessthan 1 volt. To this end, transistor O1 is chosen to be an NPN germaniumtransistor. The three components R1, R6 and Q1 form the principalcurrent path through the analyzer. The portion of the circuit comprisingthe transistors Q2 and Q3, resistors R2, R3, and R5 and capacitor C1forms the current sensing means 18. For best results, the emitter-basepotential-current relationship of transistor Q3 should have a sharp andwelldefined knee. This characteristic is found with the silicon tpetransistor and therefore O3 is by preference a silicon type of PNPconstruction.

A branch of this circuit comprising the emitter and collector of atransistor Q3 and resistor R5 connects between the terminals 56 and 58.

The base of transistor Q3 is connected via resistor R3 to the collectorof transistor Q1. The base of transistor Q1 is connected to thecollector of the transistor Q2, of PNP construction.

The requirements for this transistor are similar to transistor Q1, i.e.,it should have low saturation voltage, low emitter-base potentials andin general operate in voltage ranges less than 1 volt. For this reason,02 is desirably of the germanium type. The emitter of transistor Q2 isconnected to the collector of transistor Q1. The base of transistor O2is connected to the collector of transistor Q3. The resistor R2 connectsthe collector of transistor O2 to the output terminal 58. A capacitor C1in series with a resistor R4 is connected between the base and collectorof transistor Q3.

Although the actual operation of this circuit is to control currentpulses, for purposes of explanation, it is easier to speak in terms ofcontinuous direct currents as the operation is the same in either case.

Current flowing through resistor R1 and R6 causes a voltage drop betweenpoint 56 and point 60. This same voltage will appear between thecollector and base of transistor 03. r I

It is well known that the voltage'dropacross the emitterjunction of asilicon transistor is practically constant, regardless of the currentthrough it and that this voltage is approximately 0.7 volts. It is to benoted that the forward base emitter junction potential of transistor Q3is used in the present circuit as a reference potential.

If the voltage drop across the emitter junction of transistor Q3 shouldbe greater than 0.7 volts, transistor Q3 will rapidly increase itsconductivity and increase the voltage drop across resistor R5.

When the voltage drop across R5 increases, the conductivity oftransistor O2 is decreased. Since transistor Q2 is a PNP type, thisincreases the negative bias on Q1, and since Q1 is a NPN typetransistor, this reduces its conductivity which in turn reduces thecurrent flow through R1 and R6, until the voltage drop across the tworesistors balance the normal 0.7 volts drop across Q3. This actionoccurs in a very short time and well within the period of a single heartstimulating pulse. if, on the other hand, the voltage drop' across theemitter junction of transistor Q3 should be less than 0.7 volts,transistor Q3 will rapidly decrease its conductivity and decrease thevoltage drop across resistor R5. This will result in an increase in thecurrent flow controller.

through'Ql, again resulting in a balance between the voltage drop acrossR1 and R6 and the 0.7 volts drop of Q3. Thus, the net effect ofresistors R1 and R6 plus transistors Q1, Q2 and O3 is to stabilize avoltage drop across resistors R1 and R6 and by so doing hold a constantcurrent from input terminal 56 to output terminal 58. It will also beseen that the value of the controlled current will be adjustable by thevalue given resistor R6.

Transistor Q3 is chosen to be a silicon device because it has a largevoltage drop than germanium transistors and is therefore a more stablevoltage reference. Transistors Q1 and Q2 are-preferably germanium forminimum voltagedrop. The germanium transistors do not react as fast assilicon. in order to match the silicon transistor reaction time to thatof the germanium transistors, a retarding circuit is included. Thisretarding circuit is comprised of resistor R3, R4 and capacitor C1 whichslow down the response time of transistor Q3 so as to match transistorsQ1 and Q2. Resistors R2 and R5 serve to set the several transistors atdesirable working voltages. Resistor R3 also serves to limit the curerntflow from O3 to point 60. Without R3, the circuit may be unstable.

The device as described in detail above is designed to pass, a pulse ofnegative electricity as generated by a negative emitting pulsegenerator. If by chance a positive pulse should be passed through it,the device will not be damaged but will not control the current flow.However, if the device is to be used for controlling a positive pulse,it is only necessary to connect terminal 58 to the generator andterminal 56 to the load. When so connected, the controller becomes apositive current It has been stated that transistor Q1 should be an NPNgermanium type for best results. The device will operate quitesuccessfully if 01 is a PNP transistor provided that transistors Q2 andQ3 are NPN type. This changes the polarity of the device so that whenconnected as shown in FIG. 2, it will control positive currents passingfrom terminal 56 to terminal 58.

if transistors Q1 and Q2 are silicon, the device will 7 function buthave a higher minimum control voltage.

Also, it was stated that transistor Q3 should be a silicon type for'bestresults. The device will operate successfully even if 03 is a germaniumtransistor, but with somewhat less accuracy of control. In these cases,there may be no particular need for the retarding circuit R4 and C l.

, From this detailed description it will be seen that the variableresistor R6 forms a current selection means by which a desired pulsecurrent value can be selected.

Transistor 01 with its associated circuitry is a current controllingmeans by whichthe pulse current is controlled at the value. Finallytransistor Q3 and Q2 with their circuitry from a current sensing meansby which the feedback signal may prevent satisfactory operation of thesynchronous pacers.

It may be pointed out the the calibrations of variable resistor R6 arechosen so as to control the current through the device at selectedvalues and are indicated on the instrument .in milliampere values. Therange covered is approximately from 1 to 20 milliamperes. Typical valuesof components used in the circuit of the invention are:

R1 R6 2.5K Pot.(external) R2 20K Cl 0.001 mfd 1 RV ceramic R3-1OKQl-2N1304 R5 200K 03 2N5447 The 10 components of the circuit are bothsmall in size and low in cost. The total cost of a unit is approximately1 percent of cost of a typical stimulator or heart pacer. Thus, theunits are low enough in cost so that they can be supplied with eachstimulator without being a burden to the purchaser.

Although the general and detailed description presented above depictsthe use of the device solely with a heart stimulator, it is quiteobvious and is included in the scope of the invention that the device issuitable to determine the threshold energy limits for other areas ororgans of the human body that need artificial stimulation such asbladder, kidneys, operational muscles, etc.

Further, the current controlling circuit of the. invention has noveltyand utility in the non-medical electrical and electronics fields where asimple, quick-acting current control is required.

The device as described is assembled from individual components.However, it is a part of this invention that it can be made using thetechniques of integrated circuitry, hybrid circuitry, etc. in order toreduce its size, reduce cost or improve reliability.

Having decribed my invention and given an example of its assembly indetail as well as a normal method of use, I hereby claim:

l. A threshold analyzer having an input terminal operably connectable toan implantable stimulating pulse generating means and an output terminaloperably connectable to an implanted stimulating electrode whichcomprises:

a. a variable resistor having a first terminal and a second terminal,the first terminal being connected to I connected to the input terminalof the threshold analyzer;

d. a first fixed resistor having a first terminal and a second terminal,the first terminal being connected to the collector of the secondtransistor and the second terminal thereof being connected to the outputterminal of the threshold analyzer;

e. a second fixed resistor having a first terminal and a secondterminal, the first terminal thereof being connected to the base of thesecond transistor and the second terminal thereof being connected to thesecond terminal of the variable resistor;

f. a third fixed resistor having a first terminal and a second terminal,the first terminal thereof being connected to the base of the firsttransistor and the second terminal thereof being connected to the outputterminal of the threshold analyzer;

g. a third PNP germanium transistor having an emith. a fourth fixedresistor having a first terminal and a second terminal, the firstterminal being connected to the base of the third transistor; and

i. a capacitor having a first terminal and a second terminal, the firstterminal being connected to the second terminal of the fourth fixedresistor and the second terminal of the capacitor being connected to thebase of the second transistor.

1. A threshold analyzer having an input terminal operably connectable toan implantable stimulating pulse generating means and an output terminaloperably connectable to an implanted stimulating electrode whichcomprises: a. a variable resistor having a first terminal and a secondterminal, the first terminal being connected to the input terminal ofthe threshold analyzer; b. a first NPN germanium transistor having acollector, an emitter and a base, the collector thereof being connectedto the second terminal of the variable resistor and the emitter thereofbeing connected to the output terminal of the threshold analyzer; c. asecond PNP silicon transistor having an emitter, a collector and a base,the emitter thereof being connected to the input terminal of thethreshold analyzer; d. a first fixed resistor having a first terminaland a second terminal, the first terminal being connected to thecollector of the second transistor and the second terminal thereof beingconnected to the output terminal of the threshold analyzer; e. a secondfixed resistor having a first terminal and a second terminal, the firstterminal thereof being connected to the base of the second transistorand the second terminal thereof being connected to the second terminalof the variable resistor; f. a third fixed resistor having a firstterminal and a second terminal, the first terminal thereof beingconnected to the base of the first transistor and the second terminalthereof being connected to the output terminal of the thresholdanalyzer; g. a third PNP germanium transistor having an emitter, acollector and a base, the emitter thereof being connected to thecollector of the first transistor, the collector thereof being connectedto the base of the first transistor and the base of the third transistorbeing connected to the collector of the second transistor; h. a fourthfixed resistor having a first terminal and a second terminal, the firstterminal being connected to the base of the third transistor; and i. acapacitor having a first terminal and a second terminal, the firstterminal being connected to the second terminal of the fourth fixedresistor and the second terminal of the capacitor being connected to thebase of the second transistor.